1. Field of the Invention
The present invention relates to an outboard motor and to a method for manufacturing an outboard motor.
2. Description of the Related Art
In recent years outboard motors have come to be equipped with catalysts for cleaning exhaust gas. For example, Japanese Laid-open Patent Application 2009-97371 discloses a catalyst disposed in a U-shaped exhaust passage. Because this U-shaped exhaust passage is removable, the catalyst can readily be extracted by removing the U-shaped exhaust passage. Japanese Laid-open Patent Application 9-49424 discloses forming a catalyst chamber midway along an exhaust passage, and disposing a catalyst in the catalyst chamber. A portion of the exhaust passage is demarcated as an exhaust passage cover, which is detachably attached to another section of the exhaust passage. Because of this, the catalyst is readily removed by removing the exhaust passage cover.
However, not all outboard motors are equipped with a catalyst, and even for outboard motors equipped with the same model of engine, the decision as to whether to include a catalyst is made according to whether exhaust cleaning capability is considered more important, or other factors such as weight are considered more important. For example, if exhaust cleaning capability is considered more important, a catalyst-equipped model will be required, whereas if weight considerations are more important, a non-catalyst-equipped model will be required. In the latter case, if the catalyst is removable as in the outboard motors disclosed in the aforedescribed documents, by removing the catalyst the weight can be reduced by the equivalent of the weight of the catalyst. However, in the outboard motors disclosed in the aforedescribed document, the catalyst is simply removable temporarily for the purpose of maintenance and the like, and the structure of the exhaust pipe remains the same even with the catalyst removed. Because of this, while it is possible to reduce weight by the equivalent of the weight of the catalyst, additional reduction in weight is desirable.
Also, in catalyst-equipped models, the catalyst is disposed along the path of the exhaust pipe. Because of this, the exhaust pipe of a catalyst-equipped model has a more complex shape than that of a non-catalyst-equipped model, and the exhaust pipe is greater in length. Consequently, simply removing the catalyst from a catalyst-equipped model does not sufficiently reduce weight. Because of this, in conventional outboard motors, in cases where further reduction in weight is desired, it will be necessary to adopt different exhaust pipe structures for catalyst-equipped models versus non-catalyst-equipped models, which makes it difficult to utilize the same engine in common.